Uplink sounding reference signals (“sounding signals”) are known signals transmitted on the uplink (mobile-terminal-to-network) direction. Such sounding signals can be used by the receiver (the base station) to estimate the uplink channel quality, including the uplink channel quality for different frequency bands. The channel-quality estimates can e.g. be used by the uplink scheduler (located in the base station) to determine a suitable uplink data rate (uplink rate control) or select a suitable frequency band for the uplink transmission for a given mobile terminal (so called channel-dependent frequency-domain scheduling).
Uplink sounding signals can also be used by the receiver to estimate the timing of the received signal. Such receive-timing estimates can be used by the network subsequently to adjust the mobile-terminal transmit timing in order to time-align the receive timing of the uplink transmissions of different mobile terminals. Other uses of the uplink sounding signals are also possible.
In Long Term Evolution (LTE), as being developed by the Third Generation Partnership Project (3GPP), the uplink sounding signals can be seen as OFDM signals, implying that they consist of a number of subcarriers with suitable modulation applied to each subcarrier. 3GPP Technical Specifications (TSs) serving as useful references for additional background details include: 3GPP TS 36.211, entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”; 3GPP TS 36.213, entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”; 3GPP TS 36.321, entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification”: and 3GPP TS 36.331, entitled “Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification”.
FIG. 1 generally illustrates an OFDM context for sounding signals. As shown, a sounding signal can be characterized in the frequency domain by: the index of the first transmitted subcarrier of the sounding signal (index eight is illustrated); the number of transmitted subcarriers of the sounding signal (twelve are illustrated); the spacing between transmitted subcarriers, also sometimes referred to as the repetition factor (RPF) of the sounding signal (a spacing of two is illustrated).
The LTE uplink time-domain structure is outlined in FIG. 2. Each subframe of length 1 ms consists of two equal-sized slots of length 0.5 ms. Each slot then consists of seven symbols. Within each slot, one symbol is used as a so called (demodulation) reference signal, which is not the same as the sounding (reference) signals introduced above. Indeed, such demodulation reference signals are used for uplink channel estimation to enable coherent uplink detection. The remaining symbols in each slot are typically used for data transmission. Within each subframe there are thus two demodulation reference symbols and twelve “data” symbols.
Accordingly, if sounding signals are to be transmitted on the uplink, a subset of the data symbols, e.g. every M-th data symbol, can be replaced by sounding signals. Typically the sounding signals are not transmitted in every subframe. Instead, one data symbol in every N-th subframe is replaced by a sounding signal (consisting of a number of subcarriers according to FIG. 1). Thus, in the time domain, the sounding signal structure can be characterized as shown in FIG. 3 by: the period (measured in the number of subframes) of the sounding signal, i.e. how often the sounding signal is transmitted (a period of four subframes is shown in FIG. 3); the time offset (measured in number of subframes) of the sounding signal (an offset of two subframes is shown in FIG. 3); the position of the sounding signal within the subframe, i.e., which data symbol has been replaced by a sounding signal (not explicitly illustrated in FIG. 3).
Within the above context, different modulation can be applied to the transmitted subcarriers of the sounding signals. This modulation may differ between different terminals within a cell or different terminals in neighbor cells. As an example, for 3GPP LTE, the modulation of the transmitted subcarriers is assumed to be based on so-called Zadoff-Chu sequences that have been extended to a length equal to the number of transmitted subcarriers. For a terminal to transmit sounding signals, it needs to know the parameters used for the sounding signal transmission including: the frequency domain parameters (bandwidth, number of transmitted subcarriers, spacing between transmitted subcarriers (repetition factor), index of first transmitted subcarrier, etc.); time-domain parameters (period. offset, position within the subframe, etc.); and which modulation symbols to use for the transmitted subcarriers of the sounding reference signal.
Some of these sounding signal configuration parameters may be implicitly given, for example, by the identity of the cell in which a terminal is active. However, several of the parameters are provided (configured) by means of downlink signaling to the terminal. In many cases, sounding signals are only to be transmitted intermittently, e.g., when the mobile terminal is to transmit data on the uplink. In view of this intermittent transmission, a base station could be configured to send sounding signal configuration parameters to a terminal each time the terminal is intended to transmit sounding signals. However, that arrangement imposes potentially high signaling overhead for managing sounding signal transmissions from a plurality of terminals.